The allele combination that could occur in the zygote is Rr.
<u>Explanation:</u>
A diploid parent contributes one allele of a gene to the gamete. Thus the gametes in human reproduction will be haploid so as to maintain the diploid nature of the zygote. In the given question both parents are homozygous and the inheritance follows Mendel’s laws.
The dominant allele is represented by the letter R and the recessive allele is represented by the letter r. One parent is homozygous dominant which means both the alleles in that parent is dominant and the genotype will be RR. The other parent is homozygous recessive with the genotype rr.
The cross can be represented as
Rr Rr Rr Rr
All the progenies will have the genotype Rr and will be dominant and possess the ability to roll the tongue.